Cristo Rei is a real time commitment — fit it in only if it's a priority.
Allow Half-day with transit.
The view back at Lisbon from across the Tagus is the real draw — the whole city laid out behind the 25 de Abril bridge, best of any vantage. The honest catch: the statue itself is a plain concrete monument, it's across the river (ferry-plus-bus or car), and beyond the lift to the top there's little else. A view trip, not a sight trip.